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This paper describes the influence of fiber length of lithium-soap thickener of greases on friction under boundary lubrication conditions. Ball-on-disk sliding tests were conducted with lithium soap greases with polyol ester as the base oil. The tests revealed that the greases had a lower friction coefficient than that for the base oil. The grease with a long soap fiber structure had a lower friction coefficient than that for the grease with a short soap fiber structure. The tests with only soap fibers showed that the soap fibers had better lubricity than the base oil under the boundary conditions tested, but there was little difference in the lubricity of the longer fibers and the shorter fibers. The partial coat tests revealed that the grease with longer soap fibers was easily entrained into the contact. It was concluded that this entrainment capability of the grease led to favorable lubrication to maintain low friction coefficient.

This article describes the influence of rheological properties on the bearing torque characteristics of the lithium soap greases with five types of base oils. The greases used had different yield stress depending on the base oils even with the same thickener concentration. Measurement of bearing torque was conducted for a deep-groove radial ball bearing by using a bearing test apparatus. The bearings filled with greases initially exhibited high torque but showed gradual decrease in the torque with prolonged rotation, where the greases with higher yield stress showed larger normalized torque decrease. Observation of bearing after the rotation revealed a tendency that the greases with larger normalized torque decrease had been pushed aside in the raceway by channeling. This implied that the greases with higher yield stress tended to show channeling. On the other hand, the greases with lower yield stress circulated within the bearing by churning and showed smaller normalized torque decrease. These behaviors were explained in terms of the yield stress of the greases and the shear stress to entrain the greases into the contacts. Observation of grease structure was made with atomic force microscopy showed that the greases whose thickener network structure was distributed more densely had higher yield stress.

This paper describes an experimental study on quantitative evaluation of correlation between the structure formed by thickeners and yield stress of greases. In order to evaluate the thickener structures, untreated surface of greases was observed by using an atomic force microscope, and an index was obtained from AFM images to describe the degree of dispersion of the thickeners. The index physically represented both factors of shape and distribution of thickeners. The results for twelve types of lithium soap greases with different base oils and thickener concentrations showed good correlation between the index of the degree of dispersion and the yield stress of the greases. This relationship indicated that uniform distribution of thickeners in greases caused higher yield stress.

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